Ill-fated king of Macedonia & Greece, whose unsuccessful attempts to maintain control of the territory united by his father (Antigonus Gonatas) weakened the influence of the Antigonid dynasty in Mediterranean politics. The apex of Demetrius' career was decades before he became king, when -- as a young teen -- he defeated Pyrrhus' son, Alexander of Epirus (ca. 262 BCE), allowing his father to claim the whole southern Balkan peninsula from the Aegean & Black seas to the Adriatic. Maintaining effective political control of this mountainous region with historically independent populations, however, proved impossible for both men. For Macedonian domination united old rivals in common cause. During the last years of his long reign, Demetrius' father repeatedly had to resort to brute force to crush revolts by leagues of Greek city states.  The Macedonian strongman's death (239 BCE) gave subjects a new opportunity to claim independence.

Thus, from the moment he became king, Demetrius faced challenges on all flanks but the Macedonian home front. A formidable coalition of Hellenic leagues from both sides of the gulf of Corinth forced him to focus on attempting to regain control of Greece. Though he succeeded in reestablishing Macedonian dominance over the region around Thebes, he failed to subdue Athens or the Peloponnese.  Then, while Macedonian forces were bogged down trying to secure their southern territories, a revolt erupted on the western flank when Epirus overthrew its king & formed a Hellenic style league of independent city states. 

A decade of campaigns against uprisings on its southern & western rims left Macedonia vulnerable to the expansionist designs of the Dardani (predecessors of the Serbs in modern Kosovo) on its northern border. In 229 BCE Demetrius concentrated all his forces on countering this new threat. But the battle-weary Macedonian army suffered a disastrous defeat. Demetrius himself was severely wounded & died shortly thereafter, leaving a crumbling kingdom to his nine year old son, Philip.

References: Justin, Epitome 26.2, 28.1-3.

Other resources on line:

• Demetrius II Aetolicus - brief article in Wikipedia's growing web.

• Demetrius II - Jona Lendering outlines key events [Livius: Articles in Ancient History].

• Justin's Epitome of Pompey's Philippic History - book 26 recounts the exploits of Demetrius & his father, while his own troubles are covered in book 28 [Corpus Scriptorum Latinorum].